Your purchase order says Class 150, but the field piping is marked PN16. The installer shrugs, lines up the flanges, and starts tightening the bolts. A day later you discover the bolt holes do not quite match, or worse, the joint weeps once the line reaches operating temperature. The valve looked right, but the rating system was wrong.
That mismatch is one of the most expensive errors in industrial procurement. Valve pressure class is not just a number on a nameplate. It controls two things that must work together: how much pressure the valve body can hold at temperature, and whether its flanges will actually bolt to the pipe. In this guide, you will learn the difference between valve pressure ratings under the ANSI/ASME Class system and the European PN system, why they are not interchangeable, and how to specify the correct class for your line.
A valve pressure class is a nominal designation, not a literal pressure limit. It tells your engineering and procurement teams which standardized family a valve belongs to, so everyone knows which pressure-temperature table and which flange dimensions apply.
When you choose a class, you are really deciding two things at once:
Getting the class wrong does not only mean the valve might be under-rated. It can mean the flange simply does not fit the pipe, or it fits closely enough to be forced together and fail later. For your business, that translates to shutdowns, re-work, and safety incidents.
The two systems you will encounter most often are the ASME Class system and the PN (Nominal Pressure) system. They are both standards-based, but they grew from different engineering traditions and use different reference temperatures.
The Class series originates from the American Society of Mechanical Engineers. For valves, the governing document is ASME B16.34, which defines pressure-temperature ratings by material group. For flanges, the mating dimensions are in ASME B16.5 (NPS 1/2 to 24) and ASME B16.47 (NPS 26 and larger). Common classes are 150, 300, 600, 900, 1500, and 2500. American standard globe valves from Dingliu, for example, are offered from Class 150 up to Class 1500 to cover a wide pressure range.
PN stands for Pression Nominale or Nominal Pressure. The European valve-shell ratings are defined in EN 12516-1, and flange dimensions are in EN 1092-1. The PN series runs 2.5, 6, 10, 16, 25, 40, 63, 100, 160, 250, 320, and 400. Cast steel globe valves from Dingliu cover PN16 through PN100, which is the range most water, steam, and process plants need.
ISO 7005 creates metric equivalents for ASME classes, so Class 150 is sometimes written as PN 20, Class 300 as PN 50, and so on. Those numbers are useful for paperwork, but they do not make the flanges interchangeable. Always remember: pressure equivalence is not dimensional equivalence.
Here is the first thing procurement teams get wrong: Class 150 does not mean 150 psi. The class number is a label for a family of pressure-temperature ratings, not a direct pressure reading. The actual allowable pressure depends on body material and temperature.
Take carbon steel ASTM A216 WCB (ASME B16.34 Material Group 1.1) at 38 °C, the standard reference temperature:
For PN valves, the nominal number is closer to the rated pressure at the standard reference temperature, but it still derates with temperature. PN16 means roughly 16 bar at the EN reference condition; by 200 °C the allowable pressure for a common cast carbon steel body has already dropped.
This is why you cannot look at a nameplate, read “Class 150,” and assume 150 psi. Actual allowable pressure must be determined from the ASME B16.34 pressure-temperature tables according to material grade and service temperature.
Every pressure class loses strength as temperature rises. The ASME B16.34 pressure-temperature rating tables and the EN 12516-1 tables both show this derating curve. If your line reaches 250 °C but you sized the valve for ambient conditions, you are operating above the rating.
For a carbon steel WCB body under ASME B16.34, the Class 150 allowable pressure drops like this:
| Temperature | Class 150 Allowable Pressure (bar) | Class 300 Allowable Pressure (bar) |
| 38 °C | 19.6 | 51.1 |
| 100 °C | 17.7 | 46.6 |
| 200 °C | 13.8 | 43.8 |
| 300 °C | 10.2 | 41.4 |
| 400 °C | 6.5 | 30.3 |
Values are approximate, per ASME B16.34-2017 for carbon steel ASTM A216 WCB (Material Group 1.1). Always verify against the latest edition of the governing standard for your project.
The lesson: always select the class based on the highest operating temperature the valve will see, not the temperature on a cool morning. For steam service, this is especially important. If you are specifying globe valves for steam, the temperature derate is what separates a reliable installation from a leak.
At ambient temperature, Class 150 and PN16 are in the same ballpark for pressure. That is why people sometimes treat them as equivalents. But they are not the same, and the danger is that they look close enough to be forced together.
Small sizes sometimes happen to share similar dimensions, but you cannot assume this. You must check each nominal size against the relevant flange standard. Here are real mating dimensions from ASME B16.5 Class 150 and EN 1092-1 PN16:
| Nominal Size | ASME Class 150 (D / D1/ n-Φ) | EN 1092-1 PN16 (D / D1/ n-Φ) | Will They Mate? |
| DN50 / NPS 2″ | 152 mm / 120.7 mm / 4 -19mm | 165 mm / 125 mm / 4-19mm | No — different bolt circle (D1) |
| DN80 / NPS 3″ | 190mm / 152.4 mm / 4 -19mm | 200 mm / 160 mm / 8-19mm | No — different bolt count |
| DN100 / NPS 4″ | 229 mm / 190.5 mm / 8 -19mm | 220 mm / 180 mm / 8-19mm | No — different bolt circle (D1) |
| DN150 / NPS 6″ | 279mm / 241.3 mm / 8 -22.2mm | 285 mm / 240 mm / 8-23mm | Dangerously close — do not mix |
| DN200 / NPS 8″ | 343mm / 298.4 mm / 8 -22.2mm | 340 mm / 295 mm /12-23mm | No — different bolt count |
ASME dimensions from ASME B16.5; EN 1092-1 dimensions from Table 13 (PN16 steel flanges). Values are rounded for clarity; verify against the current standard edition.
Notice DN150: the bolt circles are only about 1.3 mm apart, and both use eight bolts. That is exactly the kind of “almost fits” situation that causes field crews to assume the flanges are interchangeable. They are not. The bolt holes will not line up cleanly, the gasket seating surface differs, and the raised face dimensions are not the same.
If your line has mixed standards, the correct solution is either a transition flange (adapter) designed for that specific Class-to-PN change, or re-standardizing one side of the joint. Forging ahead with mismatched flanges is not a solution. If your project uses wedge gate valves or other flanged valves, confirm both the pressure rating and the flange standard before the valves reach site.
| Need Valves That Match Your Piping Standard? Dingliu supplies flanged valves in both ASME Class and PN ratings. Share your design pressure, temperature, and flange standard — we will help you pick the right class. |
Use the table below only as a pressure-rating orientation. It is based on ASME B16.34 shell ratings for WCB carbon steel at 38 °C and the closest EN 1092-1 / ISO PN grade. It does not mean the flanges will bolt together.
| ASME Class | Allowable Pressure (bar, WCB @ 38 °C) | ISO 7005 Metric Equivalent | Closest EN 1092-1 PN Grade | Notes |
| 150 | 19.6 | PN 20 | PN 25 (or PN 16 with margin check) | PN16 is below Class 150; verify design pressure |
| 300 | 51.1 | PN 50 | PN 63 | PN40 is not sufficient |
| 400 | 68.1 | PN 68 | PN 63 (use PN 100 for full rating) | PN63 is the nearest grade but below Class 400; PN100 covers the full rating |
| 600 | 102.1 | PN 100 | PN 100 | Good match |
| 900 | 153.2 | PN 150 | PN 160 | EN 1092-1 has no PN150; PN160 is nearest |
| 1500 | 255.3 | PN 250 | PN 250 | Good match |
| 2500 | 425.6 | PN 420 | PN 400 | PN400 is slightly below; verify for critical service |
Pressure values are shell ratings only, per ASME B16.34-2017 for ASTM A216 WCB at 38 °C. Actual valve selection must also consider seat leakage class (API 598 / ISO 5208 / EN 12266), end connections, and flange dimensions.
The key takeaway: Class-to-PN conversion works for pressure-rating conversations and data sheets. It does not work for flange mating. If your MRO drawer has both ANSI and PN flanges, label them separately and train your field team to read the nameplate before installation.
Here is a practical workflow your procurement and engineering teams can use every time they specify a flanged valve:
Your project location also drives the choice. Plants in the Americas, the Middle East, and many oil-and-gas export facilities standardize on ASME Class. Facilities in Europe, China, and many water-utility networks standardize on PN. If your EPC is mixing regional suppliers, put the flange standard in the purchase specification in writing. This is especially true in oil and gas valve procurement, where the cost of a mis-mated flange far exceeds the cost of a few emails up front.
When you are ready to source, look at valves that are explicitly rated for both systems if your site has mixed standards. Dingliu’s flanged centerline resilient seated butterfly valve is listed for Class 150 as well as PN10 and PN16, which gives your project flexibility when the piping standard is confirmed.
| Get the Right Pressure Class the First Time Browse Dingliu’s complete range of Class and PN rated valves for water, steam, oil, gas, and chemical service. |
No. At ambient temperature, Class 150 (about 19.6 bar for WCB) is close to PN16 (16 bar) in pressure, but they are not equivalent. PN16 is lower, and the flange dimensions are different. You need a transition flange or a valve rated for the correct standard.
The class number is a label for a family of pressure-temperature ratings, not a direct psi value. The actual rating comes from ASME B16.34 tables based on material and temperature. For WCB at 38 °C, Class 150 happens to be 285 psi.
Generally no. Outside diameters, bolt circles, bolt hole sizes, and bolt counts differ. Some small sizes look close, and DN150 is dangerously close, but the standards are not designed to mate. Use a Class-to-PN transition flange or standardize one side.
At best, the valve does not fit and your crew loses a day. At worst, the valve operates above its rated pressure-temperature limit, leading to seat leakage, flange gasket blowout, or body failure. The wrong class also voids warranty and insurance coverage in many cases.
Yes. Like ASME Class, PN ratings derate as temperature rises. EN 12516-1 gives the pressure-temperature ratings for valve shells. A PN16 carbon steel valve at 200 °C is not rated for 16 bar.
Follow the piping specification of your region and end client. Americas and Middle East oil-and-gas projects usually use ASME Class. European, Chinese, and many water-utility projects use PN. Mixed-supplier projects should define the standard in the purchase order.
Class is the ASME rating system defined by ASME B16.34; PN is the European nominal pressure system defined by EN 12516-1. They use different reference temperatures, different standard flange dimensions, and different PN grade increments. Always verify both pressure rating and flange mating dimensions.
Valve pressure class is not a marketing label. It is the link between the pressure your line actually sees, the temperature your valve actually reaches, and the flange that must bolt to your pipe. Confusing ANSI Class with PN — or assuming Class 150 equals PN16 — is how procurement teams buy the right-looking valve for the wrong system.
For your business, the fix is simple: confirm the design pressure and maximum temperature, look up the correct pressure-temperature rating for the material, check the flange standard dimension by dimension, and leave a safety margin. When everyone on your team understands that valve pressure class is two decisions in one — shell rating and flange mating — you stop field surprises before they start.
If you need valves already documented for both ASME Class and EN PN service, Dingliu can help you match the right pressure class to your piping standard. Contact us with your line data and we will recommend a valve that fits, seals, and stays rated at temperature.
Tianjin Dingliu valve Industry Co., Ltd. is a high-tech enterprise specializing in the valve and automation industry.
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